The Vatican said he had been suffering from a cold on Monday morning and had decided not to read a prepared speech to a delegation from the Conference of European Rabbis to save time so he could greet them individually.
"It happens that I am not well and because of this I prefer not to read the speech but give you a copy," he told the rabbis at that event, without going into further details.
Francis had appeared to be short of breath as he greeted the rabbis at the start of the meeting.
He is missing part of one lung, which was removed when he was a young man in his native Argentina.
But hours later he presided over a lively meeting with more than 7000 children from around the world in the Vatican's audience hall, spoke clearly and did not show any visible symptoms of a cold.
He took questions from about 14 children, including some from the Middle East and Ukraine.
He responded spontaneously, often joking with them, and appeared to be enjoying himself.
A number of questions from the children concerned peace and the environment.
They held up signs reading "peace" in their languages.
The Pope had surgery on June 7 to repair an abdominal hernia.
He spent nine days in hospital and appears to have recovered completely from that operation.
He has made three trips abroad since the surgery and is scheduled to go to Dubai next month for the COP28 United Nations climate summit.
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